1. Field of the Invention
The present invention relates to a capacitor for use in various types of electronic devices, on-vehicle devices, or the like.
2. Background Art
FIG. 7 is a sectional view of a typical electric double layer capacitor. As show in FIG. 7, capacitor element 10 has hollow portion 10a. FIG. 8 is a development of a typical electric double layer capacitor element. Capacitor element 10 is formed by winding a pair of positive electrode and a negative electrode each of which has polarized electrode layers 25 formed on current collector 27 made of an aluminum foil so that the positive electrode and the negative electrode are displaced in opposite directions each other and separators 26 are interposed between the electrodes. The positive electrode and the negative electrode are electrically extracted from end faces 21b and 21c, respectively, of capacitor element 10.
As show in FIG. 7, bottomed, cylindrical, metallic case 11 made of aluminum that houses capacitor element 10 together with a driving electrolytic solution (not shown) has protrusion 11a provided on the inner bottom face thereof. Further, protrusion 11a is assembled so as to be fitted in hollow portion 10a of capacitor element 10. The end face of the negative electrode side of capacitor element 10 inserted in metallic case 11 is mechanically and electrically connected to the inner bottom face of metallic case 11 by such a means as laser welding. Wrung part 11b having a V-shaped section that is formed on metallic case 11 presses and holds the circumference of the end face of capacitor element 10 on the upper side in the drawing, from the outward direction.
Terminal plate 12 made of aluminum includes the following elements: positive terminal 12a for external connection that is integrally formed with terminal plate 12 on the outer side thereof; junction part 12b provided on the inner side of terminal plate 12 and connected to the end face on the positive side of capacitor element 10; and projection 12c fitted in hollow portion 10a of capacitor 10. The end face of capacitor element 10 on the positive side is mechanically and electrically connected to junction part 12b by joining, using such a means as laser welding.
Insulating ring 13 is disposed on the top end of wrung part 11b formed on metallic case 11. Insulating ring 13 is formed from the portion between the inner face of metallic case 11 and the outer peripheral surface of terminal plate 12 so as to lead to a part of the inner circumference of terminal plate 12. Thereby, insulating ring 13 keeps electrical insulation between terminal plate 12 and metallic case 11.
Ring-shaped sealing rubber 14 made of an insulating rubber is disposed along the circumference of the surface of terminal plate 12. By a process of forming curled portion 11c so that curled portion 11c is caught by the opening of metallic case 11 via sealing rubber 14 (generally referred to as a curling process), capacitor element 10 is sealed. The stress of sealing rubber 14 generated by wedging engagement of the tip of curled portion 11c to the top face of sealing rubber 14 is used to ensure air tightness and prevent fluid leakage of the driving electrolytic solution.
In the conventional capacitor, fluid leakage of the driving electrolytic solution housed in metallic case 11 together with capacitor element 10 needs to be prevented. For this purpose, terminal plate 12 is disposed in the opening of metallic case 11, and sealing is attained by curled portion 11c that is formed by curling the opening of metallic case 11 via sealing rubber 14 disposed along the circumference of the surface of terminal plate 12. However, in a capacitor mechanically sealed in this manner, capacitor element 10 generates heat, when the capacitor is used in a severe environment or conditions, or undergoes continuous life tests. A resulting increase in the temperature of the driving electrolytic solution increases the pressure in metallic case 11, and in the worst case, the fluid leakage of the driving electrolytic solution causes a short circuit.